One problem often encountered when designing and operating engineering components having intersecting passages is that stress concentrations arise at the intersections or transitions between the passages when the passages are under pressure. For example, in hydraulic applications such as pumps, the passages carry high-pressure fluid which acts upon the walls of the passages to create high stress concentrations, which can lead to failure of the component by fatigue close to or at the intersection. In some applications, such as common rail fuel pumps, the stresses generated can be extremely high due to the considerable pressure of fuel that is generated within the pump.
FIG. 1(a) shows part of a known pump assembly for use in a common rail diesel engine. The pump assembly includes a pump housing 10 provided with a blind bore 12 within which a pumping plunger (not shown) reciprocates, in use, under the influence of a drive arrangement (also not shown). The plunger and its bore 12 extend co-axially through the pump housing 10 with the upper region of the bore defining a pump chamber 14 for fuel. Fuel at relatively low pressure is delivered to the pump chamber 14 through an inlet passage 16 under the control of an inlet non-return valve (not shown). Fuel is pressurised within the pump chamber 14 as the plunger reciprocates within the bore 12 and, once pressure reaches a predetermined level, fuel is delivered to an outlet passage, referred to generally as 18, via an outlet valve (not shown), which extends transversely to the bore 12. The outlet passage 18 intersects with the plunger bore 12 in a recess 20 which has an enlarged diameter compared with the diameter along the remainder of the bore. The outlet passage 18 delivers pressurised fuel to a downstream common rail of the fuel injection system.
Due to the high pressures that are generated cyclically within the pump chamber 14 during the pumping cycle, one problem that may occur within the pump assembly of the aforementioned type is high pressure fatigue of parts due to the increased stress in the region of intersection between the plunger bore 12, 20 and the outlet passage 18. As the plunger reciprocates within its bore 12 and fuel is pressurised to a high level within the pump chamber 14, a pulsating tensile stress occurs within the pump housing 10 that can cause cracks to grow. The pulsating tensile stress has two main effects within the pump housing 10: hoop stress acts around the perimeter of the plunger bore 12, 20 particularly in the vicinity of the pump chamber 14, and axial stress acts along the length of the plunger bore 12, 20.
It has previously been shown that the stress concentrations at the intersection between fluid passages can be reduced by shaping the intersection at the end of one passage, for example by radiusing the intersection to reduce the presence of sharp features and thin regions of material at the intersection. FIG. 1(b) shows a cross section of the pump assembly to illustrate the radiusing of the intersection between the outlet passage 18 and the plunger bore recess 20. At its intersecting end, the outlet passage 18 includes a conical surface 22 and terminates in a blend radius 24 between the cone 22 and the plunger bore recess 20.
The Applicant's granted European Patent No. EP 06256052 describes a more sophisticated shaping of the intersection which may be used between the outlet passage and the plunger bore in a high pressure common rail to further reduce the stress concentrations where the outlet passage meets the plunger bore. The solution proposes an intersection region which flares towards the plunger bore with a generally rectangular shape, with a radius being provided on the flare to smooth the transition between the flare and the plunger bore.
Whilst these approaches have been successful in high pressure pump applications operating to a certain pressure level, at the increasingly high pressures demanded of current common rail pumps the stress concentration at the intersecting region is not reduced enough to reduce the fatigue problem appreciably.
It is an object of the present invention to provide a high pressure fuel pump, and more generally a housing for high-pressure fluid applications, in which the stress concentrations between intersecting passages are reduced further compared to known solutions.